Mixed photoinitiators, sometimes known as co-initiators, have been disclosed for use with polymerizable monomers. Co-initiators usually comprise two compounds, a light-sensitive compound usually identified as a photosensitizer, and an activator compound which in the presence of the photosensitizer is not significantly directly excited by the activating radiation, but instead forms a free radical in response to the exposure of the photosensitizer. The known classes of such activators include amines, sulfinic acid and sulfinic acid esters, sulfones, .alpha.- and .beta.-dicarbonyls such as bornanedione and acetylacetone, phosphines, phosphites, stannates and hexaarylbi-imidazoles.
An early example of a very effective co-initiator system has been Michler's ketone as the photosensitizer, admixed with benzophenone as the activator, e.g., as described in U.S. Pat. No. 3,682,641. This mixture has long been considered an industry standard. More recently, a number of aromatic carbonyl compounds have been disclosed as useful with amine activators to form co-photoinitiators of many kinds. Such mixtures are described by Ledwith, J. Oil Col. Chem. Assoc., 59, p. 157, and especially p. 162-164 (1976). The carbonyls specifically identified are phenones, including cyclic ketones, such as benzophenone, fluorenones, anthraquinones and anthrones.
Such co-initiators have been highly effective in photopolymerization compositions of various kinds. However, notwithstanding such extensive uses, there has been a continued need to achieve still faster speeds through the discovery of novel co-initiator systems. That is, faster speeds result either in reduced exposure times or reduced amounts of exposure intensity, both of which are beneficial to the user.
U.S. Pat. No. 4,147,552 issued Apr. 3, 1979, discloses 3-substituted coumarin compounds as photosensitizers for photocrosslinkable and photopolymerizable ethylenically unsaturated compounds. The patent further mentions sensitizing mixtures of the coumarins with other compounds including specifically 3,3'-carbonylbis-(7-diethylaminocoumarin) (cols. 20-21), benzene chromium tricarbonyl and a tribromomethylated polymer (Example 14), particularly for the crosslinking of polymers. Although these photosensitizing mixtures have been found to be useful, specific speed improvements from these mixtures are reported in the patent as generally resulting from the speed provided by the coumarin alone.